Characterization of a device (e.g., a transistor or other active device) adapted to operate at radio or microwave frequencies may be performed using a load-pull measurement technique. “Load-pull” refers to a technique in which the load (or source) impedance presented to a device-under-test (DUT) is modified using a tuner. A “load-pull tuner” is a passive component that may be used to present a range of impedances (or reflection coefficients) to the DUT, where the presented impedance is a function of the physical position of the tuner's internal components. In order for a load-pull tuner to present a load impedance to the DUT, for example, the tuner is connected to the output of the DUT. The tuner may then cause part of a signal transmitted by the DUT to be reflected back to the DUT with a modified amplitude and phase. The reflected signal interacts with the transmitted signal, thus modifying the operation of the DUT (e.g., the gain or the output power).
A typical load-pull tuner includes a horizontally-oriented, slotted airline and one or more mobile carriages that are slidably engaged with the slotted airline. Each mobile carriage includes a probe, which may be inserted into and withdrawn from the slotted airline. The reflection coefficient presented to the DUT, which may be expressed using S-parameters (or “scattering” parameters), is dependent upon the physical position of the probe in relation to the slotted airline. A probe's physical position may be defined by the horizontal position of the mobile carriage along the slotted airline and the vertical position of the probe above the slotted airline.
Before using a load-pull tuner to test DUTs, a characterization process is performed for the tuner. A load-pull tuner characterization process includes repeatedly moving the probe to one of a multitude of physical positions, applying a signal at a given test frequency to an input end of the slotted airline, measuring the S-parameters with a vector network analyzer (VNA), and storing the measured S-parameters for that particular physical position in a characterization table specific to that load-pull tuner. For example, a typical, non-interpolation based characterization process for a single test frequency may include measuring S-parameters at thousands (e.g., 3000-5000 or more) of probe positions. The time to complete such a characterization process may be on the order of hours (e.g., 3-5 hours or more). When the load-pull tuner is characterized for more than one frequency, the total time to complete the characterization process roughly equals the characterization time for a single frequency multiplied by the number of frequencies for which the characterization procedure is performed. As the above discussion indicates, load-pull tuner characterization processes are very time consuming, particularly when the load-pull tuner is characterized for multiple frequencies.
Various types of DUTs may be tested using a load-pull tuner, including high power amplifiers, low noise amplifiers, mixers, receivers, and small signal amplifiers, for example. An initial load-pull tuner characterization process may be performed before the load-pull tuner is first utilized to test DUTs. Additional load-pull tuner characterization processes may be performed periodically or occasionally after that point (e.g., on a monthly basis or after testing a given number of DUTs). Once a load-pull tuner has been characterized (i.e., the characterization table is complete), the load-pull tuner may be connected to the input or output of a DUT. The test protocol may specify one or more user-defined impedances to be presented at the input or output of the DUT, and the data within the characterization table may be accessed to position the carriage and probe to achieve the user-defined impedances.